Air speed and similar indicator



6 1943 x.. A. WARNER 2,315,756

AIR SPEED AND SIMILAR INDICATOR Filed Aug. 12, 1941 3 Sheets-Sheet l ANN URL m .W mw @d .m N V R a n Mm J A AN w/ @n d @v om om 0./ w @n L mmmm @N Pd www v 1 o@ @Q =Y 0H md ww mm Wj@ P7 a n M NN m @A @A A Wm Am Om-l x m @KKK N April 6 1943- L. A. WARNER 2,315,756

AIR SPEED AND SIMILAR INDICATOR Filed Aug. l2, 1941 3 Sheets-Sheet 2:55A :I n 56 I7'r 7;?. l

LOUIS AWARNER,

um I fw/W Apnl 6, 1943. L A. WARNER 2,315,756

AIR SPEED AND SIMILAR INDICATOR 3 Sheets-Sheet 3 Fild Aug. 12, 1941 Y 5859 1. Zfc@ 4Z. f HI'lll''ILJF 62 s e4 l l &0 70a 8l so 19(l 21142141 l2,u-"" \YK 15"? IIIIIII la 146 14E [l n ,wz 49 150 4L -l- T5S IMQ/MM,7156?- sa LOUIS AWARNER 14 145 146 COJAPCIS @M 5% Patented Apr. 6, 1943UNITED STATES PATENT OFFICE AIR SPEED AND SIMILAR INDICATOR Louis AllenWarner, laltimore, Md. Application August 12, 1941, Serial No. 406,569

I (c1. vs -212) 13 Claims.

This invention relates to air speed indicators in general. Moreparticularly this invention relates to indicators in which thedifference or combined action of two electrical devices, at least one ofwhich is an electrical potential producing device, is taken to producean indication of air speed or the rate of 110W.

The principal object of this invention is to provide a Pitot-static tubedevice employing a pair of piezo electric crystal devices, one in thePitot tu-be and the other in the static tube, and connecting these to anelectrical indicating device so that the difference between thepressures exerted in the Pitot tube and the static tube will beindicated.

Piezo electric crystals consisting of Rochelle salt, quartz, tourmalinand the like may be employed in accordance with this invention as willbe apparent from the following description. Crystals of this characterare adapted to produce electrical lpotentials which vary with themechanical pressure applied thereto. These crystals, particularly thoseof quartz and tourmalin are also adapted to produce electricaloscillations of frequencies which may be varied by varying the -pressureapplied to the crystals.

In this invention I provide an arrangement in which one crystal deviceis positioned in the Pitot tube and another crystal device is positionedin the static tube so that electrical potentials varying with thepressures existing in these two tubes or electrical oscillations varyingwith said pressures may be produced. These electrical potentials oroscillations are applied to indicating circuits in which indications ofair speed or ilow are obtained.

Further details of this invention are set forth in the followingspecification, claims and the drawing, in which briefly:

Fig. 1 is a cross-sectional view of the Pitotstatic tube and crystalarrangement;

Fig. 1a is a view showing another form of mounting for the crystal inthe Pitot tube:

Fig. 2 is a schematic diagram nf one arrangement of connections employedin accordance with this invention;

Fig. 3 is another schematic diagram of connections;

Fig. 4 illustrates a form of this invention in which compensation ismade in the air speed indicator for changes in atmospheric pressure;

Fig. 5 is a -modied circuit which employs oscillating crystals; f

Fig. 6 is a View of piezo electric crystal elements supported in anevacuated housing;

Fig. 7 is a diagram' of another arrangement of connections showing theuse of a pair of oscillating crystals; and

Fig. 8 illustrates a form of this invention vemploying only one crystaldevice, and bellows for positioning its electrodes.

Referring to Fig. 1 of the drawings in detail,

- reference numeral I designates the Pitot tube having open mouth IIpointed in the direction of travel. Reference numeral I2 designates thestatic tube having air holes I3 toward the rear thereof and having abullet-shaped cap I 4 at the end. Both the Pitot I0 and the static tubeI2 are attached to the supporting member I5 which is provided with oneor more pipes I6 adapted to -be attached to a strut or other inember ofan airplane. A piezo electric crystal device Il is positioned in thePitot tube and a similar 'piezo electric crystal device I8 is positionedin the static tube. The device I'I consists of two piezo electriccrystal slabs or plates I9 and these are separated by a thin metallicelectrode y 20. A pair of side electrodes 2I connected together areemployed. In cases where Rochelle salt crystals are employed,- thecrystals are as` sembled with the electrodes and a moisture im, perviouscoating of lacquer, varnish or wax 22 is applied. This is thenenclosedin a casing 23 which is fitted into the Pitot tube I0 and fastenedtherein. A metallic diaphragm 24 is attached to the mouth of the casing23 and the varyiable air pressure is transmitted through this diaphragmand the moisture impervious crystal coating to the ciystal. This varyingpressure causes the piezo electric crystal device to producecorrespondingly varying electrical potential which is applied to theconnections 25 and 26.

As seen in Fig. 2, connection 25 is connected to one of the grids of thetube 2l and connection 26 is connected to the side electrodes 3| of thepiezo electric crystal elements 29 positioned in the static tube I2. Thepolarity of the electrodes 3I is the same as that of the electrodes 2Iand these piezo electric crystal devices are in this case differentiallyconnected so that the difference of their potentialsis applied to thevacuu-m tube 21. The crystal elements 29 are similar to the crystalelements I9 and they are also provided with a moisture impervious,coating 33, a housing 32 fitted into the static tube I2, and thediaphragm 34 for transmitting variable atmospheric pressure thereto.

A winding of heatingwires surrounding the Pitot tube, and supply leadstherefor, for melting ice, is shown at 18.

The central electrode 2U is connected by means of the wire 25 to thegrid 39 of the tube 21 and the cegitral electrode 30 is connected by thewire 35 to '.he grid d'5, the grid resistor 36, the 4cathode biasresistorf andthe negative terminal of the .source of current supply 4l.It is therefore seen vthat the piezo electric crystal elements aredifferentially connected between the grid electrodes 39-5- The cathodebias resistor 43 is sliunted by a by-pass condenser 44 and is connectedbetween the cathode 38 and the negative terminal of the battery 41. Thecathode 38 may be of the lament t'ype or it may be of the indirectlyheated type. An auxiliary grid 4I connected to the positive terminal ofthe battery 41 through the resistance 42 is also provided to the tube.Anode 40 of the tube 21 is connected to the indicating device 46 throughthe resistance 49. This resistance may be dispensed With if theindicating device 45 is of sumcient impedance to be itself directlyconnected to the anode. The positive terminal of the battery 41 isconnected to the indicating device 46 and a by-pass condenser 48 isshunted across the battery 41.

In the modified arrangement shown in Fig. 3, the crystal elements I9 and29 are connected to separate tubes 5I) and 5I, respectively, and in thiscase the central electrode 20 of the crystals I9 is connected to thegrid 53 of the tube 50 by means of the wire 52. Wire 54 connectselectrodes 2| to the grid 55. The anode 56 of the tube 58 is connectedthrough the resistance 51 to the coil 58 of the differential meter 59.

Likewise the central electrode 30 of the crystals 29 is connected bywire 60 to the grid 6I of the tube 5 I' and the out-side electrodes 9|are connected by wire 62 to the grid 83. The anode 64 of the tube 5| isconnected through the resistance 85 to the other coil 66 of thedifferential meter 59. In this case a pair of anode batteries 61 and 68is illustrated.

In operation the meter 46 of Fig. 2 and meter 59 of Fig. 3 produceindications which are proportional to the difference of the potentialsproduced by the piezo electric crystal devices I9 and 29. Consequentlythese meters may be calibrated in air speed units because the piezoelectric crystal device I9 is responsive not only to the air impactcaused by the movement of the aircraftthrough the air but it is alsoresponsive to the varying static atmospheric pressure. The piezoelectric crystal device 29 on the other hand is responsive only to thevarying static -atmospheric pressure. The pressure applied to thisdevice would therefore decrease as the static atmospheric pressuredecreased.

Where desired the piezo electric crystal elements I9 and 29 and theassociated electrodes may be supported inside of evacuated containers insuch a way that the atmospheric pressure is applied for the purpose ofcompressing the crystals only along a single directional line ofapplication. This type of housing is shown in Fig. 6;"

The piezo electric elements I9 and the electrodes 29 and 2| associatedtherewith are supported by means of the supporting members 1I and 12between the plates 19 and 13a. The members 1I and 12 may be in the formof coil springs which may be compressed to a slight extent. The plates13 and 13a abut against the inside surfaces of the ends of the container15. The pressure applied to plates 13 and 13-a is transmitted bysupporting members 1I and 12 to the faces oi' the piezo electricelements I9, and a potential varying with the air pressure is produced.This container 15 may be in the form of a bellows if desired and theplates 13 and 13a may form the ends of this bellows. A heating winding14 connected in series with a thermostatic device 16 and a source ofcurrent supply is provided to maintain the temperature inside of thecontainer'15 substantially uniform. 'I'he pressure inside of thecontainer pressure but may be either more or less than atmosphericpressure as desired.

In Fig. 1a I have illustrated another form of mounting for the piezoelectric crystal devices I9 and 29. In this form of mounting one end ofthe piezo electric crystal elements I9 is substantially rigidly clampedby the member |9a which may be in the form of a ring adapted to slideinto the mouth of the tube II. This ring is also provided with a flange1b adapted to engage the pe'- ripheral portions of the diaphragm I1 sothat this diaphragm may be clamped into place thereon. 'I'he free end ofthe piezo electric crystal elements is coupled to one end of the arm I9band the other end of this arm `is pivoted at I9c. A connecting arm |10is provided between the center of the diaphragm andthe arm I9b. Thepiezo electric crystal elements I9 mounted as shown in Fig. la producean electromotive force through the ilexing, bending or torsion of theelements when said elements are subjected to a pressure producingflexing, bending or torsion. The electrodes 29 and 2| are also providedto these elements and these electrodes are connected as shown in Figs.2, 3 and 4.

Referring to the automatic compensation modification of Fig. 4 indetail, the piezo electric crystal elements I9 and 29 are in this caseconnected to control grids of the vacuum tubes 50 and 5I. respectively,and an additional control tube is provided for the purpose of varyingthe grid bias on the grid 55 of the tube 50. 'I'he control l grid 8| ofthe tube 80 is connected to the electrode 30 of the piezo electriccrystal `elements 29 and to the grid 6| of the tube 5|. The anode 62 oftube 80 is connected through the resistance 83 to the positive terminalof the battery 61a and to the differential windings 58.68 of the meter59. This anode 82 is also connected .to the grid 55 of the tube 56. Bymeans of this tube 80 and these connections the piezo-electric crystaldevice 29 which corresponds to the device 29 shown in Fig. 1, functionsto control the grid bias on the grid 55 of the tube 50.

Therefore as the atmospheric or other pressure on the crystal elements29 decreases, less negative potential is applied to the grid of the tube80 causing more current to ilow through this tube and this results in agreater potential drop across the resistance 83, so that more negativepotential is applied to the grid 55 of the tube 50, that is, the bias onthe grid 55 is increased and .the current flowing through the tube 50between the anode 59a and the cathode 50c is decreased. 'Ihe purpose ofvarying the grid bias of the grid 55 in this way is to providecompensation for reduced air pressure with increased altitude. As waspointed out above, the current through the tube 50 is decreased when thepressure on the piezo electric crystal elements 29; which are in thestatic chamber I2 of the Pitot tube, is decreased. At the same time thetotal force of the air pressure on the crystal elements.l9 which are inthe open end of the Pitot tube, is also decreased because of the lowerdensity of the air at higher altitudes. The circuit is so adjusted thatthe indicator 59 produces true air speed indications at all altitudesirrespective of variations in the air density. If desired the connection84 may be made variable over the resistance 83 so that they y circuitmay be adjusted manually by the operator.

Referring further to Fig. 4, the cathodes of the tubes 59 and 5I areconnected through suitable resistance and condenser circuits 10 and 10ato 15 need not necessarily correspond to atmospheric 75 the cathode ofthe tube 89 and the negative terminal of the source of current supply61a. The grid 63 of the tube 5| is also connected to the negativeterminal of the battery 61a either directly or through a suitableresistance which may be variable and may be used for the purpose ofadjusting the apparatus. Suitable grid leaks are also connected betweenthe grids 53 and 6| of the tubes 50 and 5|, respectively, and thecathodes thereof. The crystal elements I9 and 29 also have terminalsthereof connected together and two electrodes of the same polarity ofthe crystal elements are connected together. In the diagram illustratedthe positive electrodes of the crystal elements are connected together.This connection between the crystal elements is connected to thenegative terminal of the source of current supply 61a while the otherelectrodes of the crystal elements are connected to the grids 53 and 6|respectively.

In Figs. 5, 7 and 8 I have illustratedgf` form of this inventionemploying oscillating piezo electric crystal devices.

Referring to the oscillator arrangement of Fig. the piezo electriccrystal element I9' which corresponds to the elements I9 of Fig. 1 isconnected in series with the crystal element 3 between the grid |0| andthe cathode |02 of the oscillator tube |00. In this circuit both of thepiezo electric crystal elements are in oscillation at frequenciescorresponding to the one or more of the dimensions thereof. Thefrequencies of the crystals I9 and 3 I are preferably close to eachother. However they may be the same. In practice it is preferable toemploy crystals which produce a suitable beat frequency, Which is thedifference between their frequencies or between harmonics of theirfrequencies. In practice I prefer to ernploy beat frequencies of 100,000or 200,000 cycles so as to simplify the construction-of the series tunedcircuit |03 employing the condenser |05 and the inductance |04. Thisseries tuned circuit is connected across the inductance |06 which iscoupled to the tank circuit |01 of the vacuum tube oscillator. Therectifiers |00 and |09 are connected with their anodes tothe ends of thetuned circuit |03 and with their cathodes to the resistance ||0.Thecondensers and ||2 are connected across the resistance ||0 and thecommon terminal of lthese condensers is also connected to the tunedcircuit |03. An indicator ||3 is connected across the resistance I I0.

Referring further to Fig. 5, when the pressure on the crystals I9 and3|', or either one of them, is varied, the frequency of oscillationthereof is also varied or changed. Consequently the beat frequencyproduced by mixing the frequencies of these crystals Would also change.By mounting the crystals I9' and 3| in a Pitot static tube such as shownin Fig. l the pressure on the crystal I9', which is mounted in the openmouth of the Pitot tube, is varied in accordance with the speed of thecraft, while the pressure on the crystal 3|' is not varied in thismanner since this crystal is mounted in the static portion of the tube.Consequently the beat frequency produced between the frequencies ofthese crystals is Varied in accordance with the air speed of the craft.Since the tuned circuit |03 is tuned to the original beat frequency, theindicator ||3 will produce indications which correspond to the departureof the beat frequency from the frequency to which the circuit |03 istuned. The indicator ||3 therefore produces indications which areproportional to the air speed of the craft. The frequency of the crystaldevice I9' may be made either to increase or decrease with an increasein pressure thereon. The same is true of the crystal device 3|' so thatthe apparatus may be made to compensate foi changes in atmosphericpressure with changes in altitude.

The embodiment of my invention shown in Fig. 7 also employs oscillatingcrystals. lIn this case however crystal I9' is connected to the grid andcathode of the oscillator ||4 which is provided with a tank circuit ||5coupled to the indicator tuned circuit IIE. The crystal 3|' on the otherhand is connected between the grid and cathode of the oscillator ||1employing a tuned tank circuit ||8 coupled to the tuned circuit ||9 ofthe indicator. The tuned circuits ||8 and ||9 are tuned to the crystalsI3 in the Pitot tube and 3| in the static chamber respectively, or toharmonics or subharmonics thereof, and these tuned circuits arepreferably isolated or shielded from each other so that thecorresponding diode Arectiflers |20 and |2| function to rectify onlyvthe potentials induced into the respective tuned circuits ||8 and H9from the respective tank circuits ||5 and |38. These rectifiedpotentials are applied across the resistance |22 differentially or inopposition. The meter |23 is connected across the resistance |22 andproduces indications corresponding to the difference of the rectifiedpotentials.

Referring further to Fig. 7, an auxiliary circuit |24 is coupled throughits inductance |25 to the tuned circuit ||8. This auxiliary circuitemploys a series tuned circuit |28 tuned to the frequency of the circuitI8 and the potential across this series tuned circuit is rectified bymeans of two oppositely connected rectiers |26 and |21. A potentialcorresponding to the diierence'between the rectied.potentials is appliedacross the resistance |29 which is connected to the solenoid |30associated with the crystal I0.' 'I'he purpose of this arrangement is toemploy changes in frequency produced in the static crystal device 3| bychanges in altitude, to vary the air gap between the crystal electrode|3| and the crystal I9'. In this way the frequency of the crystal unitI9' may be shifted in accordance with changes in altitude, vthat is,changes in static atmospheric pressure. This is employed to produce trueair speed indications irrespective of changes in altitude or atmosphericpressure- In Fig. 8 I have illustrated a form of this invention in whicha single oscillatingl crystal I9' is supported on its edges by means ofclamping supports |33, between two electrodes |34 vand |35 which areconnected to, the oscillator tube |35. The crystal and its electrodesare mounted in the mouth of the Pitot tube I2' by means of thesupporting structure |36a. This structure divides the Pitot tube intotwo parts, one of which opens to the mouth of the tube and the other ofwhich forms the static part of the tube. The electrode |34 is supportedby means of the bellows |31 which opens into the forward or mouth partof the Pitot tube and moves the electrode |34 toward or away from thecorresponding face of the crystal I9' depending uponthe air pressureexerted upon the electrode and the bellows |31. The other electrode |35is also movable with respect to the crystal I9' and this electrode isslidably supported by the rod |38. the other end of said rod beingattached to the plate |30 forming one end of the second bellows |40. Thesecond bellows |40 is sealed at some definite pressure such as thelpressure at sea-level and therefore expands as it is moved to higheraltitudes. In this way this second bellows functions to move theelectrode |35 with reference to the crystal as the apparatus is taken tohigher altitudes. In the arrangement here described, the bellows movesthe electrode closer to the crystal face as the apparatus is moved tohigher altitudes. A rack |4| is attached to the the rod |38 for thepurpose of rotating the pinion |42 in accordance with the movements ofthe rod |38 and this pinion is coupled to the variable contact |43 ofthe resistance |44 for the purpose of varying the grid bias voltage ofthe oscillator tube |36 with changes in altitude. This grid bias isapplied through grid resistor |55. The cathode of tube |36 is connectedto a second variable tap |58 on the potentiometer resistance |44. Thetuned tank circuit |45 is coupled by means of small coupling condensers|46 to the series tuned circuit |41 which is tuned to the frequency ofthe circuit |45. A pair of diode rectifier tubes |48 are connected tothe series tunedcircuit |41 and to the resistance |49. An indicator |50is connected to the resistance |43 and this indicator is energized bypotential across this resistance.

This circuit o f Fig. 8 depends for its operation upon the `frequencywhich is produced in the piezo electric crystal unit I9' by varying thespacing of the electrodes with respect to the corresponding faces of thepiezo electric crystal. Thus with increasing air speed the electrode |34is brought closer to a corresponding crystal face and produces a changein the frequency of the crystal unit which change is reflected in thelndicator |50. However as the aircraft reaches the higher altitudes thesame speed will not produce a corresponding movement in the electrode|34 toward the crystal face because of the lower air densities at thehigher altitudes. Consequently the electrode |35 is brought closer tothe corresponding crystal face through the operation of the bellows |40as the altitude is increased.

As the airspeedis changed the frequency of the crystal I9 in Fig. 8changes and consequently the oscillations produced by the oscillatortube |36 also change in frequency. As a result potential diiferencescorresponding to the changes in air speed are developed across theinductance and the capacity of the tuned circuit |41, individually. Theindicating device |50 which may be in the form of a dArsonval meter,responds to the differences of the potentials rectified by therectifiers |48 and therefore indicates the departure of the oscillatorfrequency from the initial frequency.

The apparatus shown in Fig. 8 may be con sidered as a simplified form ofthe apparatus shown in Fig. 7. In Fig. 'l the static crystal 3|' isemployed to bring the electrode |3| of the crystal I9 closer to thecorresponding crystal face as the altitude increases. The forwardelectrode |32-shown in Fig. 'I may be mounted in the same manner as theforward electrode |34 shown in Fig. 8. The indicators ||3 and |23 shownin Figs. and 7 may be the same type of indicators as |50 shown in Fig.8.

Furthermore the bellows |31 and |40 (Fig. 8) are preferably made oftemperature compensated material so that variations in temperature donot affect these bellows.

Various modified forms of this invention may be made without departingfrom the spirit and scope of this invention and therefore I do notdesire to limit this invention to the exact details described andillustrated except insofar as those details may be defined by theclaims.

What I claim is:

l. A Pitot-static tube unit, comprising: a pair of hollow elements, oneof said hollow elements being open for receiving the impact of air, andthe other of said hollow elements having openings adapted to permitatmospheric pressure to exist therein, piezo electric crystal elementsin each of said hollow elements respectively, pressure responsive meansin each of said hollow elements connected to said crystal elementsrespectively for transmitting pressure thereto, and an electricalcircuit for said piezo electric crystal elements connected forindicating the difference in air pressures on said piezo electriccrystal elements.

2. An air speed indicator, comprising: a piezo electric crystal element,means for impressing the impact of air on said piezo electric crystalelement whereby said element produces an electric potential varying withthe force of the impact of air, correction means responsive toatmospheric air pressure and comprising electronic tube means forcontrolling the effect of the electrical output produced by said piezoelectric crystal element in accordance with variations in theatmospheric air pressure, and electrical indicating means connected tosaid element and said correction means for producing air speedindications controlled by the said piezo electric crystal element andsaid correction means.

3. An air speed indicator, comprising: a piezo electric crystal element,means for impressing the impact of air on said piezo electric crystalelement whereby said element produces an electric potential varying withthe force of the impact of air, an amplifier connected to said piezoelectric crystal element, means responsive to atmospheric air pressurefor varying a bias voltage of said amplifier in accordance withvariations in the atmospheric air pressure, and electrical indicatingmeans connectedV to the output of said amplifier for producing air speedindications controlled by the said piezo electric crystal element andsaid second means.

4. A Pitot-static tube unit, comprising: a pair of hollow elements, oneof said hollow elements being open for receiving the impact of air, andthe other of said hollow elements being open to permit atmosphericpressure to exist therein, piezo electric crystal elements in each ofsaid hollow elements respectively, pressure responsive Ameans in each ofsaid hollow elements connected transmitting static atmospheric airpressure to` said piezo electric crystal, and means connected to saidfirst means and to said piezoelectric crystal for producing air speedindications controlled by said first means and said piezo electriccrystal.

6. An indicating device for indicating flow of or speed through avariable density medium, comprising: a piezo electric crystaladaptedunder applied pressure to produce an electrical output having afrequency characteristic thereof variable in accordance with the flow ofor speed through a variable pressure medium, means responsive to thedynamic pressure of said flowing medium and adapted to apply saidpressure to said crystal, and correction means comprising electronictube means for changing said piezo electric crystal output frequencycharacteristic in accordance with the static pressure of said medium.v

7. An indicating device for indicating now. of or speed through avariable density medium,

comprising: a pair oi' piezo electric crystals, one ,of said crystalsbeing exposed to air pressure produced by the impact of moving air andthe other being exposed to static atmosphere, a pair of Aamplifier tubeshaving. the cathodes4 thereof connected togethen, an indicator havingdifferential windings Aconnected to the respective auxiliary tube havingits output connected for anodes of said tubes, said piezo electriccrystals i being connectedto grids of said tubes, and `an,

exposed to air impact, the other of said piezo electric'crystals beingconnected to the grid of 1 said auxiliary tube, whereby said crystalexposed to static atmosphere controls the grid bias of said ampliiiertube which isconnected tol said crystal exposed to air impact inaccordance with changes in density of the medium.

8. A Pitot-static tube unit, comprising: a pair ofhollow elements, onevof said hollow elements being open for receiving the yimpact of'air, and

the other of'said hollow elements having openings adapted rto permitatmospheric pressure to exist therein, piezo electric crystal elementsin each of said hollow elements respectively, means in each of saidhollow elements for applying to the crystal element therein the pressureexisting in said. hollow element respectively, electronic tube meanscomprising a plurality of control grids respectively connected lto saidcrystal elements, and an electrical indicating instrument connected tothe output of said tube meins@ i 9. A Pitot-static tube unit,comprising: a pair of hollow elements,` one of said hollow elementsbeing open forreceivingthe impact of air, and the other 'ofsaidff hollowelements having openings adapted to 'permit atmospheric pressure toexist therein, piezo electric crystal elements in each of saidhollowelements respectively, means in each'ofsaldliollow elements for applyingto the crystal element thereln'the pressure existing in said hollowelement respectively, an electrical circuit system comprising electrontubes and an oscillating circuit and being connected to said crystalelements, and an electrical indicating instrument connected to saidcircuit system.

10. In a compensated Pitot-static tube unit,` a first receptacle adaptedto maintain therewithin the static pressure of the ambient air, a secondreceptacle open at one end for receiving the impact of impinging air, afirst and a second piezo electric crystalA element respectively'mountsaid tube, and means actuatable by displacement ed in saidreceptacles, oscillatingv circuits-'re- `last named individual -meansfor rectifying the outputs of said' tuned circuits, and means formeasuring the difference between said rectified outputs.

11. In a compensated Pitot-static tube unit, a rst receptacle adapted tomaintain therewithin the static pressure of the ambient air, a secondreceptacle open at one end for receiving the impact of impinglng air, afirst and a second piezo electric crystal element respectively mountedin said receptacles, oscillating circuits respectiveiy connected to saidcrystal elements, tuned voutput circuits respectively connected to saidoscillating circuits and being respectively tuned to the frequencies ofsaid crystal elements, individual means for rectifying the outputs ofsaid tuned circuits, means for measuring the difference between saidrectified outputs, electrodes for said crystal elements, an auxiliarytuned circuit coupled to the oscillating circuit of the crystal inthestatic pressure receptacle. l I

means for rectifying the output; .of'saidauxiliary circuit, andelectro-mechanical means adapted under applied voltage to vary' thespacing betweena said electrode and asaid crystal in said secondreceptacle and. being connected to said rectifying means of saidauxiliary circuit.' i Y' l2. In a compensated Pitot-static tube unit, afirst receptacleV adapted to maintain tnerewithin thefs'tatic pressureof the ambient air, a second receptacle open at one end for receivingthe impact of implnging air, va common chamber attached to both of saidreceptacles, a piezo-electric crystal 1n Isaid chamber, electrodes -forsaid crystal, individual meansror displaceably mounting said electrodesrespectively for varying tne spacing of said electrodes from saidcrystal, a pair or' pneumo-mecnanical means respectively mountedin saidreceptacles and' being responsive to the pressures respectively apphedin said receptacles and adapted to cause linear displacement underdifferences in applied pressure and being respectively connected to saidelectrodes,

'and electrical means connected to said crystal and adapted' to measurethe frequency-of the outputvoltage thereof. V

- 13. In a'rcompensated Pitotv-st'atlc tube unit, a first receptacleadapted to maintain therewithin the static pressure of the ambient air,a second mountingsaid electrodes respectively for varying the spacing ofsaid electrodes from said crystal; a pair of pneumo-meciianical meansrespectively mounted in said receptacles and being responsive to thepressures respectively applied in said receptacles and adapted to causelinear displacement under dili'ei'ences in applied pressure Aand beingrespectively connected to said electrodes, an electron tube having itsinput connected to said crystal, electrical circuits including anelectrical measuring instrument connected to the output Ofsaid tube,mechanically adjustable means for adjusting the grid bias of a gridof ofthe pneumq-mechanical means mounted in said static pressure receptacleand adapted to vadjust said` grid bias adjusting means.

LOUIS ALLEN WARNER.

